I have the problem from about 2-3 YEARS!!, I sent many emails to tech support but doesn’t seem to work anything, I read emails at MacBook and when I see the iPad or iPhone aren’t the same marked as read, so you might imagine the frustration because I mostly control the emails that I want to read after so I left as “unread” but later, take the iPad at home and instead of have for instance 10 emails, shows 40, obviously 30 already read at iPhone o MacBook!!
my problem is just email, no messages
Yes, my MacBook is late 2011 with High Sierra and iPhone and iPad with the latest iOS, might think the problem is the MacBook, BUT the problem is the same if I read at iPhone and then open iPad, so without using the MacBook
the problem is intermittently, some times sync, and sometimes dont